The exploitation of marginal gas wells has necessitated an increase in the volume of proppant pumped through a frac head and associated wellhead isolation equipment during certain well stimulation operations. More than 10,000,000 pounds (4,500,000 kg) of proppant (e.g., frac sand, sintered bauxite, or ceramic pellets) mixed with a fracturing fluid such as “slick water” may be pumped down a wellbore at rates of up to 300+ bbl/minute during a multi-stage well stimulation procedure. As understood by those skilled in the art, pumping millions of pounds of abrasive proppant through a frac head at those rates causes abrasion, commonly referred to as “wash”, even if the frac head is designed to be abrasion resistant.
Frac heads are normally constructed from a frac head body of alloy steel (e.g. 4140 steel) with a central passage that provides a conduit for directing high-pressure fracturing fluids into a frac mandrel. The frac mandrel provides pressure isolation for pressure-sensitive wellhead equipment and conducts the fracturing fluid into a casing or a tubing of a well. Side entries are drilled through the frac head body to communicate with the central bore, and inlet ports are welded into the side entries. The outer ends of the inlet ports provide connection points for “frac irons”, which are steel pipes that conduct the high-pressure fracturing fluids from frac pumps to the frac head. Frac heads are generally built with 2-5 inlet ports. Each inlet port must be carefully welded into the frac head body by a skilled welder after the parts are pre-heated to 400°-600° F. to prepare them for welding. The welder builds up layers of weld metal to secure each inlet port. The weld must secure the inlet ports against 10,000-15,000 psi of fluid pressure induced by the frac fluids and violent mechanical forces transferred from the frac irons, which frequently vibrate and oscillate with significant force in response to flow obstructions and/or unbalanced pump loads. After all of the welding is completed the frac head is post-heated to 1100-1150° F. for about an hour/inch of thickness of the thickest part, and controllably cooled to below 300° F. before the welded areas are ground to a finished surface. Alternatively, the grinding may be performed before the post-heating. After complete cooling, paint is applied. All of the skilled labor, time and materials required to build the frac head makes it expensive to construct and to own.
Furthermore, when a frac head becomes worn due to wash, it has to be transported to a specially equipped machine shop to be refurbished. This may require transporting the heavy frac head hundreds or thousands of miles for repair. To refurbish the frac head, the washed surfaces have to be machined down to a consistent internal diameter to prepare them for welding, an operation known as “over boring”. If an inlet port or a bottom flange/adapter is too worn, it may have to be completely cut out and replaced with a new component. After machining, the frac head is heated (400°-600° F.) to prepare it for welding before weld metal is built up on the machined surfaces to a required thickness to restore the frac head to original specifications. Once the welding is completed the frac head must be post-heated to 1100-1150° F. for about one hour/inch of thickness of the thickest part for stress relief, and controllably cooled to below 300° F. The frac head is then re-machined to provide a smooth bore to inhibit abrasion. If any defects are discovered after machining, the entire heating, welding and post-heating processes must be repeated. Not only is refurbishing a frac head a time-consuming and expensive operation, the welded repair is never as resistant to abrasion as the original parts. Furthermore, the repaired frac head must be returned to the field, which again entails transportation expense.
In order to reduce the cost of maintaining frac heads, abrasion-resistant frac heads were invented, as taught for example in Applicant's United States Patent application number 2006/0090891A1 published on May 4, 2006. Abrasion resistant frac heads significantly reduce frac head maintenance, but cannot eliminate it. Because abrasion-resistant steels are brittle they cannot be used to line a bottom end of the central passage through the frac head, which is subject to impact and compression forces. Consequently, even abrasion-resistant frac heads require occasional maintenance besides the replacement of abrasion-resistant liners.
There therefore exists a need for a frac head that can be refurbished in the field.